Bile salts and lecithin or phosphatidylcholine form complex micelles in aqueous solution. In mammalian bile these micellar systems are essential in maintaining cholesterol in solution. Cholelithiasis occurs when there is insufficient bile salt and lecithin to keep the cholesterol in solution. Considerable success has been achieved in recent years in treating gallstone disease with chenodeoxycholic acid. The mechanism whereby this treatment solubilizes cholesterol is not completely understood. The purpose of this research therefore is to provide basic solution thermodynamic information on bile salt-lecithin solutions and their cholesterol dissolving properties. Measurements of the enthalpy of dissolution of lecithin by bile salt solutions will be completed. The vapor pressures of bile salt and bile salt-lecithin solutions will be measured to obtain the free energy of these solutions. Partial molar volumes of bile salts in pure aqueous solution and in complex micellar solution with lecithin will be measured. Thermodynamic parameters for the dissolution of cholesterol by bile salt solutions, bile salt-lecithin solutions and bile salt with added electrolyte will be determined. These data will be useful in elucidating mechanisms of cholesterol dissolution processes, and will provide information on the composition, stability and structure of bile salt-lecithin micelles. This information will help to provide a rational basis for the development of new regimens and procedures for gallstone dissolution. It is expected that solubility data and thermodynamic parameters for the dissolution of cholesterol by a variety of bile salt-electrolyte mixtures will be an important aid in developing media for dissolution of retained common duct stones after cholecystectomy.